1. Field of the Invention
The present invention relates to a method for controlling errors in a link layer during wideband wireless communication, and more particularly, to a method for controlling the errors in a wireless link layer using a simultaneous multiple copy scheme and an adaptive forward error control scheme for a real time service during wideband wireless communication. The present application is based on Korean Application No. 99-35839.
2. Description of the Related Art
In wideband communication using an optical cable, transmission error ratio is very low due to the stability of communication media. However, in the case of wireless communication, the transmission error ratio is higher than in communication through wire and the distribution of the error ratio is very variable, due to the characteristics of media. Therefore, the error controlling scheme is very important to wideband wireless communication.
The error controlling scheme in the conventional wireless link layer is classified into a forward error correction (FEC) scheme for correcting errors in a wireless channel, an automatic repeat request (ARQ) scheme for performing re-transmission when errors are detected in received data, and a hybrid ARQ scheme where the above two methods are combined with each other. The ARQ scheme will be described first.
FIG. 1 schematically shows processes of controlling errors in the link layer by the conventional ARQ method. In general, the ARQ scheme or a sliding window method is used for controlling the errors in the link layer. There are various types of ARQ schemes. Here, a selective repeat method, which is a representative ARQ scheme, will be described.
As shown in FIG. 1, a receiver 12 feeds back an acknowledgment (ACK) to a transmitter 10 when each cell (the transmission unit of the link layer) transmitted by the transmitter 10 is successfully received and feeds back a negative acknowledgment (NAK), which indicates that a cell has been corrupted, when a cell transmitted by the transmitter 10 is not successfully received. A time delay taken to receive the ACK or the NAK from the receiver 12 after the transmitter 10 transmits the cell is defined as a round trip delay (RTD) 14. The receiver 12 generally uses a checksum in order to determine that a cell has been corrupted. A cyclic redundancy code (CRC) is used for the representative method for calculating the checksum.
In FIG. 1, the receiver 12 finds error in a third cell 16 and feeds back the NAK with respect to the corresponding cell to the transmitter 10. After the RTD, the transmitter 10 which received the NAK re-transmits xe2x80x9conexe2x80x9d copy 16a of the third cell 16 to the receiver 12. Also, a corresponding cell 17a is re-transmitted with respect to a seventh cell 17 in which an error occurs. As a result, it is noted that a corresponding cell 19 is successfully received by the receiver 12.
However, errors may occur in the re-transmitted cells 16a and 18. In this case, the receiver 12 feeds back the NAK and the transmitter 10 re-transmits the corresponding cell 18a. 
In the FEC scheme, when the receiver finds an error in a cell, the receiver does not request the transmitter to re-transmit the cell, but recovers or corrects the cell. However, severe overhead is caused by encoding all information required for recovering the error and inserting the encoded information in a transmitted cell. It is difficult to apply the FEC scheme as the size of the cell increases. Therefore, a hybrid ARQ scheme, which first recovers or corrects a certain number of errors by the FEC and then determines whether an error exists by the CRC is generally used, instead of the pure FEC scheme.
FIG. 2 schematically shows the structure of a wireless cell according to the conventional hybrid ARQ scheme. The cell received through wire (a wire ATM cell), which is a protocol data unit (PDU) 20 of an upper layer, is encapsulated to a PDU 25 of the link layer for the wireless channel and then transmitted. Here, an asynchronous transfer mode (ATM) cell will be described.
A wire ATM cell 20, including a header 21 of 5 bytes and a payload 22 of 48 bytes, has a fixed length of 53 bytes. The header 21 and the payload 22 of the wire ATM cell 20 are converged as they are to a header 21a and a payload 22a of the PDU of a wireless data link control layer (wireless DLC-PDU), and then the CRC 23 and the FEC 24 in order to control the error in the wireless channel are added for constituting the wireless DLC-PDU. Here, it is assumed that the wireless ATM DLC-PDU has a fixed length of 64 bytes, as proposed currently. Therefore, when it is assumed that the length of the CRC is 2 bytes, the length of the FEC is fixed to 9 bytes. Namely, a fixed encoding ratio is used regardless of the state of a channel.
Since a time delay corresponding to at least the RTD is required for the transmitter 10 to be informed of the error in the cell transmitted by the transmitter 10 from the receiver 12 and to re-transmit the cell to the receiver 12, the number of times of the re-transmission is extremely restricted in a real time service. However, resources can be wasted without successful transmission of a cell if only one copy of a cell containing errors is re-transmitted to provide an opportunity to correct the errors. This is because the conventional method related to the ARQ error control scheme is mainly used for a non-real time service.
Since a uniform encoding ratio is used regardless of the state of a channel in the conventional hybrid ARQ scheme, the efficiency of resources may be deteriorated in a case where the conventional hybrid ARQ scheme is used in the wireless channel in which the state of the channel changes relatively often.
To solve the above problems, it is an object of the present invention to provide a method for controlling errors in a link layer using a simultaneous multiple copy scheme and an adaptive forward error control scheme, which is capable of satisfying quality of service (QOS) such as a cell propagation delay time and a cell loss ratio (CLR) for a real-time service as well as a non-real time service in a wideband wireless communication.
It is another object of the present invention to provide recording media therefor.
Accordingly, to achieve the first object, according to an aspect of the present invention, there is provided a method for controlling errors in a link layer in wideband wireless communication using an automatic repeat request (ARQ) scheme, in which a wideband wireless channel is used for communication between a first node and a second node, comprising the steps of (a) estimating the error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel using the state of a backward (a direction in which a cell is transmitted from the second node to the first node) channel, and transmitting a cell, in which a forward error correction (FEC) code having an encoding ratio that varies depending on the estimated error ratio is included in a protocol data unit (PDU) of a wireless link layer, through the forward channel and (b) re-transmitting the copy of a cell transmitted in the step (a), when feedback information that indicates that an error exists in the cell transmitted in the step (a) is received through the backward channel.
The PDU of the wireless link layer in the step (a) preferably has a fixed length regardless of the variable encoding ratio of the FEC code.
In the step (b), a plurality of copies of the cell to be transmitted in the step (a) are preferably re-transmitted using a simultaneous multiple copy method in the same frame.
According to another aspect of the present invention, there is provided a method for controlling errors in a link layer in wideband wireless communication using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node, comprising the steps of (a) estimating the error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel through the state of a backward (a direction in which a cell is transmitted from the second node to the first node) channel, (b) determining a ratio in which the PDU of an upper layer is converged to the PDU of a wireless link layer on the basis of the error ratio of the forward channel estimated in the step (a), (c) transmitting the cell of the PDU of the wireless link layer formed according to the convergence ratio determined in the step (b) through the forward channel, and (d) re-transmitting the copy of the cell transmitted in the step (c) when feed-back information which indicates that an error exists in the cell transmitted in the step (c) is received through the backward channel.
The step (a) preferably comprises the steps of (a1) receiving a cell through the backward channel, (a2) determining whether an error exists in the cell received in the step (a1), and (a3) calculating the ratio of the number of cells determined to have errors in the step (a2) to the number of cells received in the step (a1) during a predetermined time period, and estimating the ratio as the error ratio of the forward channel.
In the step (b), when N and M are first and second positive integers and N is smaller than M, the convergence ratio is preferably determined to be 1:1 when the error ratio estimated in the step (a) is less than a predetermined threshold value and the convergence ratio is preferably determined to be N:M when the error ratio estimated in the step (a) is equal to or more than the predetermined threshold value.
According to another aspect of the present invention, there is provided a method for controlling errors in a link layer in wideband wireless communication using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node, comprising the steps of (a) estimating the error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel through the state of a backward (a direction in which the cell is transmitted from the second node to the first node) channel and determining the ratio in which the payload of the PDU of an upper layer is converged to the payload of the PDU of a wireless link layer on the basis of the estimated error ratio, (b) forming a cell to be transmitted through the forward channel by converging the header of the PDU of the upper layer to the header of the PDU of the wireless link layer, adding a checksum and a FEC code with respect to the header, converging the payload of the PDU of the upper layer to the payload of the PDU of the wireless link layer according to the convergence ratio determined in the step (a), and adding a checksum and the FEC code with respect to the payload, (c) transmitting the cell formed in the step (b) through the forward channel, and (d) re-transmitting the copy of the cell transmitted in the step (c) when feedback information which indicates that an error exists in the cell transmitted in the step (c) is received through the backward channel.
According to another aspect of the present invention, there is provided a method for controlling errors in a link layer in wideband wireless communication using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node, comprising the steps of (a) partitioning the error ratio of the wideband wireless channel into a plurality of sections and estimating the error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel to belong to one of the plurality of predetermined sections through the state of a backward (a direction in which the cell is transmitted from the first node to the second node) channel, (b) determining the convergence ratio in which the payload of the PDU of an upper layer is converged to the payload of the PDU of a wireless link layer on the basis of the error ratio of the forward channel estimated to belong to one of the plurality of sections in the step (a) and forming a cell according to the convergence ratio, to be transmitted through the forward channel, (c) newly adjusting allocated bandwidth when the convergence ratio determined in the step (b) changes due to the change of the section to which the error ratio estimated in the step (a) belongs, (d) transmitting the cell formed in the step (b) through the forward channel, and (e) re-transmitting the copy of the cell transmitted in the step (d) when feedback information which indicates that an error exists in the cell transmitted in the step (d) is received through the backward channel.
According to another aspect of the present invention, there is provided a method for controlling errors in a link layer in wideband wireless communication using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node, comprising the steps of (a) estimating the error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel through the state of a backward (a direction in which the cell is transmitted from the second node to the first node) channel and determining the ratio in which the PDU of an upper layer is converged to the PDU of a wireless link layer on the basis of the estimated error ratio, (b) transmitting the cell of the PDU of the wireless link layer formed according to the convergence ratio determined in the step (a) through the forward channel, (c) receiving feedback information on the cell transmitted in the step (b) through the backward channel, and (d) re-transmitting a plurality of copies of the cell transmitted in the step (b) when feedback information that indicates that an error exists in the cell transmitted in the step (b) is received in the step (c).
In the step (d), a plurality of copies of the cell transmitted in the step (b) are preferably re-transmitted in the same frame using a simultaneous multiple copy method.
According to another aspect of the present invention, there is provided a method for controlling errors in a link layer in wideband wireless communication using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node, comprising the steps of (a) estimating the error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel through the state of a backward (a direction in which a cell is transmitted from the second node to the first node) channel and determining the ratio in which the PDU of an upper layer is converged to the PDU of a wireless layer on the basis of the estimated error ratio, (b) transmitting the cell of the PDU of the wireless link layer, which is formed according to the convergence ratio determined in the step (a) through the forward channel, (c) receiving feedback information on the cell transmitted in the step (b) through the backward channel, (d) newly adjusting additional bandwidth required for re-transmitting a plurality of copies of the cell transmitted in the step (b) in the same frame using a simultaneous multiple copy method when feedback information that indicates that an error exists in the cell transmitted in the step (b) is received in the step (c), and (e) re-transmitting a plurality of copies of the cell transmitted in the step (b) in the same frame using a simultaneous multiple copy method.
According to another aspect of the present invention, there is provided a method for controlling errors in a link layer in a wideband wireless communication using an ARQ scheme, in which a wideband wireless channel is used for communication between nodes, comprising the steps of (a) forming a cell by converging the PDU of an upper layer to the PDU of a wireless link layer, (b) transmitting the cell constituted in the step (a), (c) receiving feedback information on the cell transmitted in the step (b), and (d) re-transmitting a plurality of copies of the cell transmitted in the step (b) in the same frame using a simultaneous multiple copy method when feedback information that indicates that an error exists in the cell transmitted in the step (b) is received in the step (c).
To achieve the second object, according to an aspect of the present invention, there is provided computer readable recording media in which programs for executing the steps of (a) estimating an error ratio of a forward (a direction in which a cell is transmitted from the first node to the second node) channel through the state of a backward (a direction in which a cell is transmitted from the second node to the first node) channel and transmitting a cell, in which a FEC code having a variable encoding ratio according to the estimated error ratio is included in the PDU of a wireless link layer, through the forward channel and (b) re-transmitting the copy of the cell transmitted in the step (a) when feedback information which indicates that an error exists in the cell transmitted in the step (a) is received through the backward channel, by computers, are recorded, in wideband wireless communication for controlling errors in a link layer using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node.
According to another aspect of the present invention, there is provided computer readable recording media in which programs for executing the steps of (a) estimating the error ratio of a forward (a direction in which a cell is transmitted from the second node to the first node) channel through the state of a backward (a direction in which a cell is transmitted from the second node to the first node) channel, (b) determining a ratio to which the PDU of an upper layer is converged to the PDU of a wireless link layer, on the basis of the error ratio of the forward channel estimated in the step (a), (c) forming a cell to be transmitted through the forward channel by converging the PDU of the upper layer to the PDU of the wireless link layer according to the convergence ratio determined in the step (b), (d) transmitting the cell formed in the step (c) through the forward channel, (e) receiving feedback information on the cell transmitted in the step (d) through the backward channel, and (f) re-transmitting the plurality of copies of the cell transmitted in the step (d) using a simultaneous multiple copy method in the same frame when feedback information that indicates that an error exists in the cell transmitted in the step (d) is received in the step (e), by computers, are recorded, in wideband wireless communication for controlling errors in a link layer using an ARQ scheme, in which a wideband wireless channel is used for communication between a first node and a second node.